LU-10985 mdt: properly handle unknown intent requests

LU-10985 mdt: properly handle unknown intent requests

Invalid intent requests should be rejected early on,
so the later code does not make any assumptions about
validity of various structures like extended pills
and such.

Lustre-change: https://review.whamcloud.com/32237
Lustre-commit: 6a39600f641cc3e179b0149af5ff17ba44d2319f

Change-Id: I74de899ae6cea7017ae8ca9a8c488ca801a38c5d
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Minh Diep <minh.diep@intel.com>
Reviewed-on: https://review.whamcloud.com/32521
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>